I am having issues with my freshwater acqurium tank. It had been running fine for 4 months or so with no real issues, and all of a sudden, my fish have slowly started to die off, one by one (neon tetras). It is then, that I discovered that my KH is nearly 0 and my pH was floating around 5. Ph out of my tap is in the 6.5-7 range, and kh is 0 as well. I wanted tips on how to increase my buffering capacity. Do I use crushed coral, or baking soda...do i use chemical ph up once i have a good buffer or do I use rock. Any help would be much appreciated!! Oh and I think my cycle has slowed as I am seeing very few nitrates and some minute levels of ammonia.

For everyone's advantage, I'm going to ask a few more questions, that you will be asked anyway.

1. How big is your tank
2. How often do you do water/filter changes
3. Is it planted, or not.
4. Have you used any chemicals in it thus far, (could be wearing off , and if so could cause your spike)
5. How did you cycle the tank, before adding the fish ( with/w out fish, ammonia/food)
6. What and how often do you feed them

Those are the questions I can think of, off the top of my head, you may be asked a few more by others. But once we know more about your tank we can try to narrow down what caused the problem and how to fix it.

But for the safe side, it never hurts to do a partial water change, to help alleviate the problem.

Over time, Biological process in aquariums can cause pH to fall but this usually doesn't happen if weekly water changes and vaccuming the gravel are done on a regular basis. Vaccumning removes organic substances and water changes help buffer the ph with minerals found in the new water while also removing dissolved substances.
Another reason for pH to fall and Kh to not register is by using water that comes through domestic water softners. These water softners remove minerals that help buffer the water and are often not recommended for use with tropical fish.
Yet another possible factor could be particular test kit. You might want second opinion if using strip style tests.
I would keep an eye on ammonia levels for they could very well continue to increase. At Ph values much below six,, beneficial bacteria has hard time sustaining itself and some claim the nitrification processs where bacteria feeds on ammonia and nitrites,all but ceases.
If none of the above in your view could be contributing to the sudden falling pH and no peat or driftwood have been used in the tank,, then perhaps coral chips in a nylon pouch would be the way to go to increase alkalinity. You will have to expieriment with thhe amount needed in a seperate container. Once you get the desired pH or kh then use that water for water changes each week.
I am not an expert but perhaps I have given you some things to consider. I will say that attempting to alter the ph in aquariums by those who like myself, aren't experts,, kills more fish than many care to admit. If you still have pH from tap of 7.0 or more,, That is the water I would use for keeping fish and for water changes.

1077 has answered and I concur with all of it. Just a comment from my personal experience, as I have the same issue.

Tap water here is 0 degrees dGH and kH, and pH 6.8-7.0. My tanks once set up slowly lower in pH, as 1077 explained its a natural biological thing, and when they get to where I want them (just above 6 is OK, I have all soft acidic water fish) I put about 2 tablespoons of dolomite in a nylon bag in the top filter chamber. For 10 years this kept the ph consistent 6.2 to 6.5 with the diurnal variation (I have heavily planted aquaria and a diurnal pH fluctuation is natural).

Dolomite is a limestone base gravel that aquarists use in marine tanks and sometimes African rift lake cichlid and livebearer tanks, since these all require harder, alkaline (basic) water. Use very little, and in something like a bag so you can add/remove it, as once it is mixed in the substrate (another method, not so good) it is difficult to remove. I understand coral chips and marble chips also work the same, but have not used those myself. A small bag of dolomite gravel is not expensive.

You can also raise GH by adding magnesium sulphate (pure Epsom Salts) to the water. I've used this previously and am experimenting with it in one of my tanks now actually [won't get into the why]. The only thing is that you need to work out the amount as it raises the GH quite suddenly, and keeps it there for at least a week; the partial water change dilutes it obviously, so it has to be replaced each water change. I think for your purposes the dolomite works better.

In my tanks, the dolomite alone in the filter of a 115g aquarium raises the GH by 20ppm and it is constant. The trick is finding the correct amount. Start with very little, 2 teaspoons for a 10g tank, in a small nylon bag in the filter if this is possible (your type of filter) or hang it next to the filter return. It takes a few days, so leave it for a week, measure the pH (and GH if you can) after a couple of days and then monitor it. Partial water changes will have little effect on dolomite treatment, since your tap water is higher in pH. In a few weeks you should see the pH stabilize. If it hasn't gone up at all after 3 weeks, add another tsp of dolomite. The change is gradual so no harm to the fish (procided you don't use too much of course).

Do not use pH adjusting chemicals, with no hardness they will act very quickly and fluctuating ph is very stressful on fish. Plus chemicals in general do not belong in fish tanks; some fish have a strong sensitivity to such things, but all fish do not benefit from chemicals.

With wild caught fish from SA or SE Asia hardness would be of very little importance, but many of the fish commonly available today are now commercially raised in tanks or outdoor ponds, and often in water closer to tap water in most areas (neutral to slightly alkaline, some GH and kH depending upon location). When fish lke neons that have ben commercially raised for years now are placed in what should be perfect water from their native habitat, they becomes stressed and can die from the shock or cumulative effect over time. I have wild caught cardinals and tank-raised cardinals, and it is not surprising that the tank-raised fish are having health problems whereas the wild are not; this is why i am now working on raising my GH every so slightly, so as not to jeopardize the wild fish (I have wild and tank-raised fish of several species together in the same aquaria) but assist the tank-raised. So far it seems to be improving.

The aquarist is one who must learn the ways of the biologist, the chemist, and the veterinarian.[unknown source]

Something we all need to remember: The fish you've acquired was quite happy not being owned by you, minding its own business. If you’re going to take it under your wing then you’re responsible for it. Every aspect of its life is under your control, from water quality and temperature to swimming space. [Nathan Hill in PFK]